It is known that semiconductor lasers, such as laser diodes, may be used for measuring distances as described by: G. Beheim et al, "Range Finding Using Frequency-Modulated Laser Diode", Applied Optics, Volume 25, No. 9 (1986).
A laser diode typically has an optical cavity comprising two opposing polished ends (called facets) each having a known index of refraction and having a light amplifying medium therebetween. Light is generated inside the diode cavity by passing electric current through the p-n junction of the diode (typically using ends of the diode other than the facets). The light inside the diode cavity is reflected from a first (e.g., front) facet to a second (e.g., rear) facet in a repetitive manner, thereby providing stimulated emission and the well known laser action. Typically, the front and rear facets are partially transparent (i.e., not 100% reflective). Thus, some light exits the laser from both the front and rear facets. The amount of light exiting an uncoated facet is determined by its index of refraction.
The behavior of a laser diode, as is known, can be significantly affected by external optical feedback, i.e., feedback of a portion of the laser output light back into the laser cavity from an external reflecting surface, as described in the article: R. Lang et al, "External Optical Feedback Effects on Semiconductor Injection Laser Properties", IEEE Journal of Quantum Electronics, Volume QE-16, No. 3 (March 1980). A laser diode together with an external reflective surface, e.g., a reflective target, can be viewed as a compound or coupled laser cavity consisting of the diode cavity and an external cavity formed by the reflective target and the laser diode facet facing the target (e.g., the front facet). The distance from the laser to the external surface must be no longer than one-half the coherence length (the distance over which the photons remain in-phase) of the output light because the light must remain coherent over the entire distance traveled (i.e., out to the target and back to the laser). Coupled-cavity effects in conventional lasers are well known, as described in U.S. Pat. No. 4,550,410 entitled "Coupled Cavity Laser" to Chenausky et al.
It is also known that if the current through a laser diode is changed from one level to another, the optical frequency that the laser diode operates at (or "lases" at; also called the "free running" frequency) will change in response thereto. More specifically, when the current is increased, the wavelength of the laser diode gets longer and, thus, the frequency that it operates at decreases. This occurs, as is known, because the temperature of the laser diode material changes with current, which causes a change in the index of refraction of the material, which causes a change in the cavity optical path length L.sub.d (also called effective diode cavity length) which is the product of the index of refraction of the laser material and the physical length of the laser cavity.
It is also known that as the optical operating frequency of the laser changes, the intensity of output light emitted from the facet not facing the target will exhibit ripples or undulation pulses (also called "mode-hops"). These intensity pulses are due to coherent interference within the laser diode between the light reflected from the target (that reenters from the facet facing the target) and the light inside the laser diode (provided the distance from the laser to the target stays fixed). Pulses occur, as is known, at laser operating frequency intervals equal to the frequency difference between consecutive external cavity modes: EQU c/2L Eq. 1
where c is the speed of light and L is the distance from the front facet to the target. It is also known that these pulses can be readily detected by differentiating the light intensity signal emitted from the rear facet.
If the target is an integral number of laser diode cavity optical path lengths L.sub.d from the laser diode, maximum constructive coherent interference occurs, and the peak amplitude of the output intensity pulses are a maximum. Similarly, if the distance from the target to the laser diode is a non-integer multiple of L.sub.d, the peak amplitude of the pulses are reduced due to destructive interference of the reflected light, but are still measurable. Thus, the peak amplitude of the intensity pulses varies with the distance but are still measurable independent of whether or not the target is an integer multiple of L.sub.d from the laser, as described in Lang et al.
In known laser diode distance measurement experiments, such as that described in Lang et al and Beheim et al, a known photo detector and accompanying electronics have been used to measure the light emitted from the rear facet of the laser and to produce a voltage signal indicative thereof. The voltage signal from the detector is analyzed to determine distance information.
The distance L from the front facet to the target is given by the known equation: EQU L=Nc/2.DELTA.F Eq. 2
where N is the number of intensity pulses (or external cavity "mode-hops") that occur over the laser frequency change .DELTA.F; c is the speed of light; and .DELTA.F is the change in laser frequency that occurs due to the change in laser diode drive current. L is very much (many orders of magnitude) larger than the optical path length of the laser diode cavity. Thus, the distance L to the target may be determined by merely counting the number N of "mode-hops" that result from the laser frequency change .DELTA.F. The theoretical resolution in distance measurement, as is known, is the distance corresponding to one "mode-hop" or: EQU .DELTA.L=c/2.DELTA.F Eq. 3
Thus, if .DELTA.F=50 GHz, then .DELTA.L=3 mm, which is good resolution, as discussed in Beheim et al.
However, numerous problems occur in attempting to implement laser diode coherent interference-based distance measurement in a real-world environment. First, one of the largest problems with coherent light detection is speckle noise. Speckle noise, as is known, is an optical noise generated as a consequence of the scattering of coherent light when it hits a surface that is not perfectly flat (on a wavelength scale). Also, all targets exhibit a finite amount of surface vibration, which increases speckle noise. Furthermore, if the target is rotating, such as a helicopter rotor blade (like that described in copending U.S. patent application Ser. No. 07/665,061, filed Mar. 6, 1991), some wobble will exist which also increases speckle noise. This noise interferes with coherent detection and can cause the optical intensity to drop-off periodically as a function of distance to the target, thereby preventing intensity measurement at certain distances (i.e., measurement drop-out) and making distance measurement unreliable.
Second, Beheim et al discusses using an up-down ramp (positive sloped ramp followed by a negative sloped ramp) current waveform to drive the laser diode; however, an up-down ramp produces a DC shift in the differentiated waveform which varies as a function of the number of pulses seen over a given ramp time, thereby causing nonuniform pulse amplitudes, which can lead to inaccurate distance measurement. Furthermore, an up-down ramp can generate intensity pulses in two opposite polarities, requiring electronics that detects both polarities. Also, the up-down ramp waveform introduces inaccuracy due to the abrupt change in the waveform slope (from positive to negative).
Also, it is known that if the target is in motion (i.e., has a finite velocity), additional pulses (with similar amplitude characteristics as those discussed hereinbefore) will appear on the laser output signal due to a known Doppler effect (called the Doppler frequency Fd). This occurs whether or not the laser diode drive current (i.e., the laser optical frequency) is changing with time. Consequently, the total number of pulses per sweep of the drive current (herein called a ramp cycle) emitted from the laser is related to the distance (Fx) and the velocity (Fd) of the target. More specifically, for an up-down current ramp drive signal, when the drive current is increasing and the target is moving toward the laser, the number of pulses seen on the feedback is: Fx+Fd. Conversely, when the drive current is decreasing and the target is moving toward the laser, the number of pulses seen is: Fx-Fd. If Fd is greater than Fx (i.e., the target is moving faster than a certain speed) the result of the relation: Fx-Fd, is negative and the direction of the pulses on the decreasing slope will change polarity, thereby requiring the electronics to compensate for this occurrence. This requires the electronics to be much more complex and costly, or that velocity constraints be placed on the target. If the target is moving away from the laser, the above relationships are the same except the sign of Fd is reversed for both cases.